1. Field of the Invention
The present invention relates to fine flaky boehmite particles which are useful as the starting material for the preparation of fine flaky alumina particles or as a filler suitable for a pigment for coating materials, a filler for rubbers and plastics and a coating material for paper making, and a process for efficiently preparing the boehmite particles by hydrothermal treatment.
2. Description of the Prior Art
Up to this time, various fine boehmite particles and various processes for the preparation thereof have been known and examples of such processes include one which comprises conducting pyrolysis of aluminum hydroxide as a starting material in the atmosphere to remove part of the water of crystallization, thereby forming a boehmite phase, and one which comprises conducting hydrothermal treatment of aluminum hydroxide in a hermetically sealed vessel to form a boehmite phase.
Although the boehmite particles prepared by the above processes can be finely ground mechanically by the use of, e.g., a ball mill, into fine particles having a size of the order of submicron, these processes are problematic in that the obtained particles tend to become globular and fine flaky particles are difficult to obtain.
Of the above processes for preparing fine boehmite particles, the pyrolysis process has another problem in that when the aluminum hydroxide to be used as the starting material is contaminated with fine particles, no boehmite phase is formed, but chialumina (.chi.-Al.sub.2 O.sub.3), which is a transitional alumina, is formed to lower the yield of boehmite particles. On the other hand, the hydrothermal treatment process has another problem in that the formed boehmite particles are in the form of a tight aggregate of rhombic prisms and cannot form fine flaky particles, even when they are finely ground.